Assuming you are using one of the more recent versions, click on tools->options. If you do not see a menu with tools click alt it will appear next to the firefox label in the top left of your screen.

Inside options click on 'advanced' the last tab. Here you will see options to clear your cached web content (copies of websites you've visited for speed) as well as offline web content and user data which I don't think is populated without explicit commands to do so.

LOL, yeah, it's pretty amazing how they think they can "reason" with you. - Sidewalker, speaking of advocates for sexual deviancy.